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Solution-processable, soft, self-adhesive, and conductive polymer composites for soft electronics - Nature Communications
Results . Design and fabrication of SACPs for soft electrical interfaces . Electrical interfaces for soft electronics require both high bonding stability and a low interface electrical impedance to maintain a good electrical signal transmission. The bonding performance relies on the adhesion strength and modulus (Fig.? S1 ), which may cause delamination, wrinkles, or slippage, resulting in the collapse of interfacial electrical connections (Fig. S1, i–iii) 32 , 33 . Hence, a soft electrical interface of low modulus, strong adhesion, and high electrical conductivity is capable to address this issue (Fig.? S1, iv ). SACP is one of the suitable candidates for soft electrical interfaces, owing to the low modulus, strong adhesion, and high conductivity. SACPs consist of three components, i.e., SMS (citric acid and cyclodextrin with a molar ratio of 10:1), elastic polymer networks (chemically crosslinked PVA networks with GA), and conductive polymers (PEDOT:PSS) (Fig.? 1a ). Briefly, citric acid, cyclodextrin, PVA, and GA were successively added to the aqueous solution of PEDOT: PSS. A homogeneous mixture was obtained and served as a “coffee-ring” free ink (Fig.? 1b ). SACPs showed various applications in soft electronics owing to the following aspects (Fig.? 1c–d ). First, supramolecular interactions occur from hydrogen bond interactions of a large number of carboxyl groups and hydroxyl groups in SMS 34 . Meanwhile, SMS can form hydrogen bonds and electrostatic interactions with PSSH units and positively charged PEDOT, respectively (Fig.? 1a ) 35 . Such interactions significantly inhibit the aggregation of PEDOT chains (Figs.? S2 and S3 ), thus increasing the degree of freedom of the polymer chains 36 , and thereby improving their mechanical flexibility. Second, PVA polymer networks, which are formed due to the selectivity of chemical crosslinking of GA with PVA and β-CD (Figs.? S2a–b and S4 ), are constructed in PEDOT: PSS composites to obtain large yet reversible stretchability. Notably, the addition of dopants may inevitably cause plastic deformation 23 . Weak internal networks may cause greater plastic deformation upon stretching 28 . PVA networks solve the problem of irreversible deformation 37 , resulting in good recovery of SACPs (Fig.? S5 ). Third, owing to the existence of abundant hydroxyl functional groups and charged molecules, SACPs exhibit strong interface adhesion on substrates due to the synergistic effects of several weak interactions at the interface 38 . Fourth, the components are all soluble in water and capable of forming a homogeneous, stable, and viscosity-tunable aqueous ink (with viscosity on the order of 10 3 –10 4 ?mPa?s, Fig.? S6 ), rendering it particularly suitable for solution processing towards the scalable fabrication of flexible devices 39 . Mechanical and electrical performance of SACPs . Previous studies indicate that PEDOT:PSS composites possess a compromise between low modulus and high conductivity. Here we demonstrated low modulus and high conductivity can be simultaneously obtained in SACPs. We first investigated the doping effect of SMS on the mechanical properties of SACPs. Stress-strain curves of the SACP films with different mass ratios of PEDOT:PSS were measured by tensile testing. The mechanical property of SACP with a?36.3% mass ratio of PEDOT:PSS is hard with fracture strain of 154%, young’s modulus of 5.9?MPa, and fracture stress of 14.8?MPa. While, the mechanical property of SACP with a 3.6% mass ratio of PEDOT:PSS is soft with fracture strain of 736%, Young’s modulus of 401.9?kPa, and fracture stress of 1.2?MPa. We observe a significant decrease in Young’s modulus and an increase in stretchability after doping. As the mass ratio of PEDOT: PSS decreases from 3.6% to 0.9%, the elastic modulus and fracture stress of SACPs gradually decrease to 56.1?±?13? kPa and 593.2?±?178.2?kPa. While the fracture strain slightly increases up to 700% at low PEDOT:PSS content (mass ratio <3.6%) (Fig.? 2a, b and Fig.? S7a ). The modulus of SACP can be tuned compatibly to that of skin tissue 16 . SMS doping provides an effective strategy to soften the PEDOT: PSS composites in a wide range. Fig. 2: Mechanical and electrical performance of SACP films. a Stress-strain curves. b Young’s modulus, fracture stress, and fracture strain of the free-standing films with different PEDOT:PSS loadings. c Images of the films (with PEDOT:PSS mass ratio of 3.6%) presenting little residual strain even after being stretched to 400% of strain. d Stress-strain curves of the films (with PEDOT:PSS mass ratio of 3.6%) under cyclic loading-unloading tests. e Magnifying stress-strain curves between 0–120% for checking residual strain after stretching. f Conductivity of the films with different PEDOT:PSS loadings. g Conductivity of the films (with PEDOT:PSS mass ratio of 3.6%) with different thicknesses. h Relationships of Young’s modulus and conductivity versus fracture strain of a variety of PEDOT:PSS composites reported in references and this work. The error bars show the standard deviations from at least three tests. Full size image Not limited to SMS doping, the mechanical properties of SACPs can also be further improved with PVA networks as well as the water drying process. Residual strain is one key parameter for soft materials when they need to be stretched in practical applications. By introducing a PVA elastic network, we lower the strain residual of PEDOT:PSS composites. SACPs with a 3.6% mass ratio of PEDOT:PSS show a large range of reversible stretchability (400%) (Fig.? 2c ), and the residual strain is <50% even at a large tensile strain (Fig.? 2 d and e ). Not only strain residual, but we also observed the improvement of elastic resilience as the increase of concentration of the GA crosslinker (Fig.? S7b and Supplementary Video? 1 ). In addition, the mechanical performance is significantly changed as water evaporation time varies (Fig.? S8 ), due to the impacts of water molecules on the hydrogen bond interactions in SMS. Along with the tuning of mechanical properties of SACPs, the electrical conductivity also shows remarkable controllability ranging from 1 to 37?S/cm. We investigated the conductivity of the SACPs at a different mass ratio of PEDOT:PSS (Fig.? 2f ). Interestingly, we observed an increase in the conductivity at the small mass ratio and a gradual decrease at the large mass ratio, with a maximum conductivity of 37?S/cm at a 36.3% mass ratio. Given that the SMS and PVA networks are insulating, the conductivity of SACPs compliance percolation theory 22 , 40 . Therefore, there is a trade-off between mechanical flexibility and conductivity. Besides, the as-made composites can be prepared as conductive films. Interestingly, the as-made SACP films with different thicknesses (4–340?μm) by spin-coating (for thin-film) and casting (for thick-film) methods showed good uniformity in electrical conductivity (Fig.? 2g ). According to the dynamic electrical resistance variations test (Fig.? S9 ), the relative electrical resistance changes <1 at a strain of 100%, and the variation values remained constant under repeated strain-releasing cycles. This result indicates that the SACP films present good stability of resistance at a tensile strain within 100%. Figure? 2h shows a comparison of Young’s modulus and conductivity of PEDOT:PSS and their composites. Intrinsic PEDOT:PSS has a large young’s modulus (>500?MPa), low stretchability (<5%), and low conductivity (<1?S/cm) (Fig.? 2h , black square) 41 . And it is difficult to reconcile mechanical and electrical properties. For example, ionic liquid-doped PEDOT: PSS composites have high conductivity (>100?S/cm mostly) (Fig.? 2h , purple region) 36 , 42 , 43 . However, such composites exhibit a high young’s modulus (>10?MPa), poor stretchability (fracture tensile strain <200%), and serious plastic deformation. Recently, to produce elastic PEDOT:PSS composites with low young’s modulus and high conductivity, ionic liquids plasticizers were used to adjust the microstructure of PEDOT complexes. For example, polyurethane and ionic liquid-doped PEDOT:PSS composites have been achieved at a modulus of 7?MPa and electrical conductivity of 140?S/cm 44 . In contrast, PEDOT: PSS hydrogel was developed with some features of low modulus (<1?MPa) and large ductility (200–600%) (Fig.? 2h , green region) 26 , 27 , 45 . Although hydrogels are of great significance in the application of bioelectronics, the conductivity of hydrogel materials is much poorer (0.01–23?S/cm) as compared to their counterparts. What is more, the strain residual is typically larger than 100% for hydrogels and there is also serious plastic deformation. The present SACP exhibits relatively good electrical properties (up to 37?S/cm) and much superior mechanical properties, i.e., lower modulus (up to 56?kPa), low strain residual (<50% at 500% tensile strain), smaller plastic deformation, and larger stretchability (up to 700%), rendering it one promising candidate for soft electronics. Taking into account the compromise in mechanical flexibility, conductivity, and interface adhesion (in following discussions), SACPs with PEDOT:PSS mass ratio of 3.6% presented suitable mechanical property (modulus of 401.9?kPa) and conductivity (3.79?S/cm) meet the requirements of bioelectrode. Interface adhesion performance . Traditional PEDOT:PSS films will break apart during repeated bending cycles, and the poor interface adhesion makes them easy to peel off from the substrate (Fig.? 3a ). The present SACP film exhibits high adhesion performance on PI and elastic Ecoflex substrates (Fig.? 3 b, c and Supplementary Video? 2 ). The adhesion force of the SACP film on the substrate can tolerate a variety of load conditions (hanging, pulling, and lifting) (Fig.? 1c and Fig.? S10 ), and even applicable to biological tissues such as the liver surface of pigs (Fig.? 3d ). Fig. 3: Interface adhesion performance of SACP films. a – b Images of pure PEDOT:PSS ( a ) and SACP ( b ) films on PI substrate, insert: magnifications of adhesion interfaces. c Image of peeling off PI film from the adhesive film. d Image of adhesion between adhesion film and liver of a pig (the liver surface was cleaned by absorbent paper). e Illustration (left) and image (right) of 180° peeling tests of SACP films. f Measured peeling force per width of SACP films with different PEDOT:PSS loadings on PI substrates. g Summary of adhesion strength of SACP films on a wide variety of substrates. h Images of the bending state of substrates by releasing pre-stretched SACP film-adhered substrates. i Adhesion stability of SACP films after being stored in the constant temperature and humidity (temperature 30 °C, humidity 40%) for different days. j Adhesion strength of cyclic adhesion tests. k Schematic illustration (left) and image (right) of lap shear tests. l Lap shear strength of SACP films with different thicknesses. m Summary of lap shear strength of SACP films on different substrates. The error bars show the standard deviations from at least three tests. Full size image We first investigated the adhesion strength of SACP by applying a 180-degree peeling test (Fig.? 3e ). The results show that the adhesion depends on the concentration of PEDOT: PSS in SACP films. The adhesion force increases as the PEDOT: PSS content decreases, and the interfacial adhesive force is about 400?N/m for the PEDOT:PSS content at 0.9% (Fig.? 3f ). Importantly, the self-adhesive film shows fast bonding ability (30?s) (Fig.? S11 ), owing to multiple interactions such as hydrogen bonds, ionic interaction, and Van der Waals’ interactions 46 . Notably, the SMS of citric acid and cyclodextrin has good interfacial adhesion (several MPa) 34 . More importantly, the SACP film shows substantially high performance of adhesion on diverse substrates (>150?N/m for PI; >120?N/m for PEEK; >100?N/m for Al, Cu, and PET; >80?N/m for PTFE; >30?N/m for PDMS) (Fig.? 3g ). The differences in interfacial adhesion forces of various polymer substrates were mainly dependent on molecular polarization strength of polymer chains and variations in chemical structures 10 , 47 . The high adhesion strength on PI or PEEK substrate can be ascribed to the strong dipole interactions of –C=O groups of their polymer primary chains. Meanwhile, PE or PDMS substrate has low interface adhesion strength due to their weak dipole interactions of polymer primary chains. Besides, the interface roughness and interface energy of substrate are also important factors affecting adhesion strength. The interface adhesion strength is even better than that of commercial test tapes (Fig.? S12 ). The strength of interface adhesion on the substrate is strong enough to overcome the bending stress of different substrates (TPU, PC, PI, and TPFE) (Fig.? 3h and Supplementary Video? 3 ). Besides, the SACP films show remarkably high stability in storage and repeated usage. We did not observe any obvious decay in adhesion strength by storing the film (temperature at 30 °C, humidity of 40%) for 30 days (Fig.? 3i ). Although we observed the adhesion strength reduced by 25% for a second time of the adhesion on PI substrate, the adhesion performance remains stable in the subsequent 100 cycles (Fig.? 3j ). This feature of repeated adhesion performance may be originated from physical interaction on the interface, avoiding irreversible damage of delamination 47 . We further investigated the lap shear strength of SACP on diverse substrates. The lap shear test was conducted by depositing the SACP film (<50?μm in thickness) on diverse substrates (Fig.? 3k ). We observe that the shear adhesion force increases from 0.6 to ~1?MPa as the thickness of the film increases from 7 to 20?μm as shown in Fig.? 3l , and finally reaches a stable adhesion strength (over 1.2?MPa) from 28 to 43?μm, which is larger than that of the PU adhesive layer 30 . Such a strong lap shear strength can also be visualized by the lifting up of a 5?kg weight by the overlapped film (Fig.? 1c ). Like the adhesion strength, the lap shear strength shows a similar trend relating to different substrates (Fig.? 3m ). However, lap shear adhesion strength is much greater than the peeling adhesion strength, indicating a significant adhesion anisotropy in the adhesion performance of the SACP thin film. The strong lap shear adhesion strength is of significance to improve the stability and durability of SACP thin-film devices. Solution processability . SACP can be processed by various solution-processing techniques owing to its excellent stability, homogeneity, and “coffee ring” free patterning capability. Although the contact angle of the SACP solution shows a slight fluctuation on different substrates (Fig.? S13 ), the fluid dynamics of the droplets and the resultant patterns on the substrate are consistent (Fig.? S14 ). As a proof-of-concept, we demonstrated the fabrication of SACP films and patterns by various solution process techniques as well as their applications in transfer printing and flexible ACEL devices. Figure? 4a–c shows the solution-processing fabrication of SACP films and patterns, e.g., microfluid molding, drop-casting, and spin-coating. SACP films can be deposited in microchannels, leading to the formation of “U” shape films on the walls of microchannels (Fig.? 4a and Fig.? S15 ). The combination of drop-casting and laser patterning techniques enables the fabrication of SACP-patterned Ecoflex. As shown in Fig.? 4b (i)–(iii) , the as-made patterned film showed no delamination from Ecoflex substrate under repeated stretching, indicating high adhesion stability of SACPs on the soft dynamic surface. In addition, a transparent conductive film with a maximum area of over 8?×?8?cm 2 can be obtained by spin coating (Fig.? 4c ), which can serve as transparent electrodes in photoelectric devices 48 . We also demonstrated the fabrication of large-size (13?×?19?cm 2 ) transparent yet adhesive thin film on PET substrate by using Meyer rod coating method, and the corresponding SACPs conductive film can be used as electrodes of a large-area ACEL device with an area of 9?×?14?cm 2 (Fig.? S16 ). The transmittance of the SACP film is close to 70–95% at 550?nm (Fig.? 4e ). The as-made SACP film exhibits square resistance of about 1000 Ω/□ with the light transmittance at 90% (Fig.? S17a ) and good bending stability (Fig.? S17 b, c ). We further demonstrated the transfer printing of the SACP patterns onto PI from PTFE substrates according to their difference in adhesion (Fig.? 4d and Supplementary Videos? 4 ). We also demonstrated the fabrication of SACP films on diverse substrates (PDMS, PET, PI, TPFE, glass, and copper) by spin coating (Fig.? 4f ). The transparent conductive film on the PET substrate was not broken even pasted by commercial test tape several times and no obvious electrical resistance changes were observed (Fig.? 4g and Fig.? S17d ). The self-adhesive conductive film can be cut and adhered face-to-face to form a mechanically durable and electrically conductive connection (Fig.? 4h and Fig.? S17e ). Similar to the superiority of self-healing organic conducting film on electrical recovery after mechanical damage 49 , 50 , this SACP film may also open a way to electrical recovery. Moreover, the SACP film exhibits a lower impedance as compared with the traditional PEDOT:PSS (Fig.? S18 ). Fig. 4: Solution processing and patterning of SACP patterns and thin films for flexible or even transparent electrodes. a Microfluid molding. b Drop casting. c Spin coating. d Transfer-printing of SACP patterns for lighting up an LED lamp from Ecoflex to PI substrate enabled by the difference of adhesion strength. e UV–vis spectrum and images of different transmittance SACP films. f Images of SACP films on various substrates by spin-coating. g Images of SACP films under repeating peeling tests by commercial test tapes. h Images of adhered PET film under bending and twisting. i Image of alternating current electroluminescence (ACEL) devices by applying SACP thin film as transparent electrodes. j Scanning electron microscope (SEM) image of the cross section of the ACEL devices. k Images of the ACEL device (i) under folding (ii), bending (iii) and wrinkling (iv). l Images of the ACEL device immersed in water for 24?h under bright (i) and dark (ii) backgrounds. Full size image We fabricated ACEL devices based on the SACP transparent thin-film electrodes by spin-coating. In the experiment, the bottom SACP electrode (7?μm), the light-emitting layer (ZnS:Cu particles and PDMS, 75?μm), the top SACP electrode (7?μm) and the PDMS encapsulation layer (35?μm) were spin-coated on the PET substrate (5?μm) layer by layer, and thus ACEL thin-film device was obtained (Fig.? 4i ). The thickness of ACEL film does not exceed 150 μm (Fig.? 4j ). ACEL devices have exceptionally high performance in flexibility and can withstand various deformations including bending, kneading, folding even multiple folds (Fig.? 4k , Fig.? S19 a–c and Supplementary Videos? 5 ). Not limited to the ultra-high flexibility, the ACEL device is highly stretchable when using an elastic substrate, and the emission intensity increased at low strain then decreased at high strain, because the electrical field is varied by the trade-off between the resistance and space of electrode under strain (Fig.? S19 d–h ). Such a thin and light structure allows the working ACEL device to flutter in the wind. In addition, the good interface adhesion performance renders it a good packaging, and as such the ACEL device can work stably even in water for 24?h (Fig.? 4l ). Physiological electrical potential monitoring . The SACP film is an ideal candidate for bioelectronics owing to its low modulus, high adhesion strength, and good conductivity 15 , 51 . The SACP film can conformally adhere to human skin (Fig.? 5 a–c ). The adhered film deforms synchronously when skin suffers from various mechanical deformations, i.e., stretching, compressing, and distorting (Fig.? 5d ). As a proof-of-concept, SACP films are applied as conductive interfaces between human skin and electromyograph (Fig.? 5e and f ). The biological EMG signals of a human hand are monitored at different gripping forces. The results reveal that the performance of the adhesive and conductive film electrode is comparable to a commercial Ag/AgCl electrode (Fig.? 5g ) and the other conductive materials in literatures 11 , 13 , 15 . By applying 5, 10, 15, and 20 kilograms of forces to the grip strength meter, the SACP electrode can monitor the steady increase of the EMG signals (Fig.? 5h and i ). In addition, the EMG signals do not show significant attenuation even when the SACP electrodes had been used ten times (Fig.? 5j and k ). Fig. 5: Skin adhesion and electromyography (EMG) monitoring of human exercises by using SACP electrodes. a – b Adhesion of films on joints of fingers ( a ) and skin of arm ( b ). c Images of skin texture patterned on an SACP film after peeling off from the skin. d Images of SACP films adhered on the skin under distorting. e Interface connection between skin and flexible printed wires. f Image of an EMG test: SACP film electrodes were attached to the skin near the muscle, and gripping forces were displayed by a dynamometer. g Comparison of EMG signals by using commercial Ag/AgCl electrode (red line) and SACP film (black line). h – i EMG signals ( h ) and peak-to-peak value ( i ) under different gripping forces. j – k Stability of EMG signals ( j ) and corresponding peak to peak values ( k ) under repeated using test. The error bars show the standard deviations from at least three tests. l Detection of body exercises from EMG signal and the corresponding images (inset), jumping (i), squatting (ii), dumbbell lifting (iii), and push-up (iv). Full size image We can also monitor various human-body activities by mounting the SACP electrodes on different muscle positions (Fig.? 5l and Supplementary Videos? 6 ). For example, by adhering it to leg muscles, we can monitor body motion by recording EMG signals of jumps and squats (Fig.? 5l (i) and (ii)). We can record two characteristic EMG signals during a one-time jump or squat motion. For jumps, the EMG signals of jumping up were larger than that of touching down. While, the EMG signals of squatting down and standing up are almost the same, in the squat-up motion. Besides, we also recorded completely different EMG signal characteristics between dumbbell lifting and push-ups by adhering self-adhesive conductive films to arm muscles (Fig.? 5l (iii) and (iv)). The EMG signals generated by continuous arm muscle contraction can be recorded. Integrations of EMG sensor and ACEL arrays . The outputs of EMG signals can be further harvested to trigger the ACEL arrays 52 . The physical movement, especially the strength, can be directly visualized and monitored by ACEL devices. To validate this concept, we designed and fabricated an integrated system for the visualization of muscle training by ACEL. The system consists of two units, i.e., EMG monitoring and displayable ACEL array (Fig.? 6a ). The ACEL array is fabricated by spin coating (Fig.? 6b ). A set of EMG monitoring electrodes (Fig.? 6c , (i)) attached to the forearm transmits the biopotential of the hand muscle to the EMG sensor (Fig.? 6c , (ii)), and then the EMG signal is converted into the corresponding value through a signal analysis and control unit (Fig.? 6c , (iii)). In accordance with EMG signals, the pixels of ACEL arrays are turned on/off depending on the threshold values, realizing the visualization detection of EMG signals of muscle motion (Fig.? 6c , (iv)). As shown in Fig.? 6d , by applying different levels of grip strength, the corresponding EMG signals are detected. After signal filtering analysis and processing, a set of control signals are generated to control the on/off of ACEL arrays. By setting appropriate switching thresholds, the ACEL arrays display according to the strength of gripping (Supplementary Videos? 7 ). SACPs serve not only the bioelectrodes for harvesting the EMG signals but also the adhesive transparent thin-film electrodes for ACEL devices. This study demonstrates a concept of the direct visualization of the physiological electrical signals of the muscles during physical movement and paves the way to develop wearable bioelectronic devices that can visualize the activity strength of our daily life in a real-time fashion. Fig. 6: Demonstration of an integrated EMG visualization system with an array of ACEL devices. a Schematic summary of the system with ACEL display controlled by EMG sensors. b ACEL display with patterned lighting arrays assembled by the transparent conductive film. c Four functional units in the system: (i) conductive film electrode for EMG signals transformation, (ii) commercial EMG sensors for data recording, (iii) commercial single-chip microcomputer for ACEL control, and (iv) flexible ACEL devices for EMG signals display. d Patterns of flexible ACEL display turning on and off based on the level of EMG signals. Full size image .
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